The present invention relates to electrodeless light sources excited by high frequency power and, more specifically, to techniques and apparatus for optimizing the transfer of power from a source to the lamp.
Historically, there have been three methods of exciting discharges with electrodes. The first uses the discharge as a lossy part of either the capacitance or inductance of a tank circuit. A second method is to place the lamp in the path of radiation from a directional antenna. A third method uses a resonant cavity which contains the lamp and a device for matching the cavity impedance to the source and transmission line. An example of a device according to this third method may be found in "Microwave Discharge Cavities Operating at 2450 MHz" by F. C. Fehsenfeld et al., Review of Scientific Instruments, Volume 36, Number 3, (March, 1965). Another example of a light source utilizing a resonant cavity is described in the U.S. Pat. No. 3,787,705 to Bolin.
All of these methods have disadvantages which limit their suitablility for use in a commercial light source such as a replacement for the conventional incandescent light bulb. An electrodeless lamp undergoes a large variation of impedance from the starting to the running conditions; at starting, the impedance is a lossy open circuit, while during the running condition, the lamp impedance is typically a few hundred ohms, having also a complex impedance component as well as a predominant real impedance component. For efficient operation, an electrodeless light source must have efficient power transfer from the source to the lamp regardless of the instantaneous lamp impedance. It would be highly advantageous to form a source of light that provides the proper impedance matching in an automatic fashion so that it could be used as a commercial light source.